Reducing Power Demands

Brian Johnston

C omputer simulation played a key role in the design of a new spunbond attenuator that has
the potential to reduce the energy required at a given production volume, providing a substantial
reduction in operating costs. Spunbond fabrics are manufactured directly from polymer in one
continuous process. The purpose of the attenuator in this process is to stretch and orient the
continuous fibers and lay them down on a moving collection screen.

One of the primary goals in attenuator design is to achieve the highest fiber velocity for a
given compressed air power level. In many situations, this makes it possible to use lower-cost
equipment. In every situation, however, it reduces energy costs and noise levels. The power
required for the process can usually be lowered by reducing velocity variations in the attenuator
cross-section. This is difficult to achieve using conventional experimental methods, however,
because it is impossible to measure velocity to the required level of accuracy.

Engineers from Hills Inc., West Melbourne, Fla., overcame this problem by using computational
fluid dynamics (CFD) to determine the effect of design variables - such as dimensions, internal air
distribution and compressed air entry angle - on the distribution of velocity and other flow
variables. Based on the understanding they gained, the engineers used computer software to develop
a large number of new design possibilities and evaluated the performance of each, finally iterating
to a new design that offers dramatic improvements.

Hills designs, develops and manufactures machinery and technology for the man-made fiber
industry. Hills engineers recognized the potential of CFD to improve the design process. CFD
provides fluid velocity, temperature, and other relevant variables throughout the solution domain
for problems with complex geometries and boundary conditions. As part of the analysis, a researcher
may change the system geometry or the boundary conditions and view the effect on fluid flow
patterns, temperatures, or the distributions of other variables. Hills engineers use Fluent CFD
software from Fluent Inc., Lebanon, N.H., because its graphical user interface greatly reduces the
amount of time and training required to set up the analysis. Fluent's parallel processing
capabilities take full advantage of workstations with multiple processors to reduce computational
time to a degree that is almost linear with the number of processors available.

Hills Inc. has a spunbond pilot line at its facilities in West Melbourne, Fla., which is
used by the company for developmental work and customer trials.

Design ChallengeThe biggest challenge in attenuator design is that the fiber is accelerated very quickly to a
high speed - from 0 to 5,000 meters per minute or more in one second or less. This means any
measurements taken in the device would alter the flow enough to make the measurements virtually
useless. The flow patterns are critical, however, because the compressed air used to accelerate the
fabric is the second-highest cost input to the process after materials. Production cost is of
enormous importance in this high-volume, low-margin industry, so equipment designers such as Hills
are continually trying to improve their product by reducing power consumption, among other things.
Without being able to visualize the flow patterns and detect variations that reduce efficiency,
they are left with a process that is largely trial and error. Because it is very expensive to build
and test a prototype design, the degree to which the design can be optimized is limited.

Schematic of a typical spunbond process with a belt collector

Validating CFD AccuracyTo validate the ability of the software to accurately model conditions within an attenuator,
engineers simulated a well-known classical attenuator described in a patent filed in 1972 by Asahi
Kasei. The CFD model accurately predicted the performance of this well-understood design. For
example, the predicted flow patterns in the injection zone, where compressed air enters the fiber
channel, suggested the existence of fiber tangling and roping due to eddy backflow. The model also
predicted that approximately 25 millimeters below the injection zone, a smooth velocity profile
develops that continues to the exit. These predictions match the performance seen in actual
implementations of this design. Hills engineers also simulated more recent slot attenuator designs
that improve on the original Asahi Kasei patent, and the models predicted smoother entry of the
compressed air along the wall, reducing the likelihood of fiber tangles and ropes relative to the
early Asahi Kasei design. This correlates well with the performance actually seen with these
designs.

Confident in the ability of CFD to predict attenuator performance, Hills engineers have begun
using the simulation tool to provide insights that allow design refinements that would not be
possible otherwise. In one recent design, the scalloped appearance of air velocity contours
predicted by CFD revealed a slight periodic nonuniformity of the air velocity due to some upstream
features. The effect was so slight - ± 2 percent at a constant Z height - that it could not be
measured with the best instruments available. Based on this information, Hills engineers optimized
the internal airflow distribution passages in order to reduce the velocity variations. After many
design iterations performed by altering the CFD model and rerunning the analysis, engineers were
able to achieve near-perfect uniformity. They also used CFD to evaluate forces due to air pressure
acting on the internal surfaces of the aspirator body parts in order to ensure that mechanical
deflections were within acceptable levels.

Optimizing The Downstream AreaHills engineers also used CFD to optimize the critical area downstream of the attenuator,
where the air and fiber flow at very high speeds. The objective in this region is to slow the
fibers down and lay them on the moving collection screen in a planned manner to produce the desired
fiber orientation and fabric properties. In early design iterations, engineers observed high shear
between the jet core and the entrained air, allowing unstable eddies to form and grow. It was
apparent that these eddies would cause the fiber to become nonuniform in the flow stream before
striking the screen. Being able to visualize the problem made it relatively easy for the engineers
to change various design parameters to develop a solution.

The appearance of spunbond fabric also can be affected by the distance between the attenuator
and the collection screen. As the distance increases, the nonuniformities in the fabric become more
pronounced. Very short forming distances can lead to other problems, however. For example, when the
high-speed jet impacts on the collection screen, it can disturb previously deposited fibers. Hills
engineers used CFD to develop and optimize the formation systems in these critical areas.

As a result of being able to visualize fluid flow and pressure throughout the attenuator
regions, the engineers were able to substantially improve the performance of a recent custom
attenuator design. In the past, they would have developed a few prototypes based on previous
experience and used the one that worked best. With CFD, they were able to develop dozens of
software prototypes, determine the flow velocity and pressure at every point in their flow domain,
and iterate to an optimized solution at a much faster pace.

The most noticeable improvement in the new design is that it makes better use of the momentum
of air to improve energy efficiency and flow uniformity. This lowers the initial cost of the
equipment, reduces the cost of energy required to operate it, and also lowers noise pressures,
improving working conditions for the operators.